Background
The authors report that "earthworms ingest large quantities of organic materials that are mixed and excreted as casts (Parmelee et al., 1990; Martin and Marinissen, 1993; Jegou et al., 1998) and improve stable macroaggregation (Guggenberger et al., 1996; Marinissen and Hillenaar, 1996; Scullion and Malik, 2000)," as has also been found by van Rhee (1977), De Vleesschauwer and Lal (1981) and McKenzie and Dexter (1987).  In addition, they note that "the retention of organic C in soil is becoming more important since the rise in atmospheric CO2 and global warming are recent concerns," and that "earthworms are known to play a role in aggregate formation and soil organic matter (SOM) protection."  However, they say "it is still unclear at what scale and how quickly earthworms manage to protect SOM."  Hence, they conducted a pair of experiments designed to investigate this question.

What was done
In the first experiment, soil aggregate size distribution together with total C and ¹³C were measured in three treatments - control soil, soil + ¹³C-labeled sorghum leaf residue, and soil + ¹³C-labeled residue + earthworms - after a period of 20 days incubation, where earthworms were added after the eighth day.  In the second experiment, they determined the protected C and ¹³C pools inside the newly-formed casts and macro- and micro-soil-aggregates.

What was learned
It was determined that the proportion of large water-stable macroaggregates was on average 3.6 times greater in the soil-residue samples that contained earthworms than in those that lacked earthworms, and that the macroaggregates in the earthworm treatment contained approximately three times more sequestered carbon.

What it means
Bossuyt et al. state that "earthworms were found to form a significant pool of protected C in microaggregates within large macroaggregates after 12 days of incubation," thereby demonstrating the rapidity with which earthworms perform their vital function of sequestering carbon in soils when plant residues become available to them.  With these results, we can thus expect to see the rapid development of the many ecological benefits reported under Earthworms in our Subject Index, as well as the magnification of those benefits that is provided by atmospheric CO2 enrichment.

References
De Vleesschauwer, D. and Lal, R.  1981.  Properties of worm casts under secondary tropical forest regrowth.  Soil Science 132: 175-181.

Guggenberger, G., Thomas, R.J. and Zech, W.  1996.  Soil organic matter within earthworm casts of an anecic-endogeic tropical pasture community, Columbia.  Applied Soil Ecology 3: 263-274.

Jegou, D., Cluzeau, D., Balesdent, J. And Trehen, P.  1998.  Effects of four ecological categories of earthworms on carbon transfer in soil.  Applied Soil Ecology 9: 249-255.

Marinissen, J.C.Y. and Hillenaar, S.I.  1996.  Earthworm induced distribution of organic matter in macro-aggregates from differently managed arable fields.  Soil Biology & Biochemistry 29: 391-395.

Martin, A. and Marinissen, J.C.Y.  1993.  Biological and physico-chemical processes in excrements of soil animals.  Geoderma 56: 331-347.

McKenzie, B.M. and Dexter, A.R.  1987.  Physical properties of casts of the earthworm Aporrectodea rosea.  Biology and Fertility of Soils 5: 328-332.

Parmelee, R., Beare, M.H., Cheng, W., Hendrix, P.F., Rider, S.J., Crossley, D.A. and Coleman, D.C.  1990.  Earthworms and enchytraeids in conventional and no-tillage agroecosystems. A biocide approach to assess their role in organic matter breakdown.  Biology and Fertility of Soils 10: 1-10.

Scullion, J. and Malik, A.  2000.  Earthworm activity affecting organic matter, aggregation and microbial activity in soils restored after open cast mining for coal.  Soil Biology & Biochemistry 32: 119-126.

van Rhee, J.A.  1977.  A study of the effect of earthworms on orchard productivity.  Pedobiologia 17: 107-114.